Various types of tools are presently in use which measure distance, with most of them satisfactorily performing a specific task. Rulers are rigid and are useful as a straight edge for drawing straight lines but cannot be shortened or expanded. Tape measures are flexible so the tape can be “hooked” at one end of the work and pulled tightly to achieve the accuracy over a short or long distance. Tape measures are mainly used in taking outside measurements.
Problems with tape measures arise when attempting an inside measure. The hook must be held firmly against one side of the enclosure while the case is moved to the other side. This requires pushing the hook against the side, which usually results in the collapse of the blade. Even if the blade doesn't collapse, it will probably sag, thereby distorting the measurement. Add to this the possibility that the case (which is part of the measurement) is not precise, or the hook sometimes being bent, or the blade collapsing when measuring vertically (as in a floor to ceiling measurement), then it is simple to see that the tape measure, although valuable for taking outside measurements, fails to perform when measuring inside places like cabinets, doors, windows, floor to ceiling etc. Also, a long inside measurement often requires assistance.
Therefore, in order to insure an accurate inside measurement, what is needed is a blade that does not collapse when it is pressed firmly against a wall or some other object and does not sag causing a distorted measurement when extended over long distances.
Some of the measuring tools exist partially solve the above problem. They are called “measuring sticks”, or the longer and heavier version called “measuring poles”. These are generally square or round telescopic tubes of variable sizes and weight with “measure” markings on them. They consist of a case which holds a telescopic extension piece (or pieces) shorter than the case with each successive piece being a few inches shorter than the previous. Neither of these tools or even a caliper is able to measure deep inside an enclosure or inside measures which are narrower than the length of the measuring tool's case.
Also, it is obvious that when such an assembly is telescopically extended horizontally, its own weight will cause the telescopic tubes to sag and. If the members of the assembly are not tight within each other, the telescopic parts will sag even more. The solution therefore is to not only make the extension piece or pieces as lightweight as possible but also have as few members as possible.